1. Technical Field
The present disclosure relates to ultrasonic tools and, more particularly, to an ultrasonic tool, or insert, for use by dental professionals for dental treatments and procedures.
2. Background of Related Art
Ultrasonic dental tools, or inserts, generally include four basic parts. The primary part comprises a laminated stack of a magnetostrictive material, which is activated at its frequency of resonance to generate sufficient mechanical power. The second part is a specially shaped tip that makes contact with the treatment area. This tip provides access and adaptation to the treatment area. The third part is an acoustic transformer, often referred to as a connecting body, which connects the laminated stack to the tip. The fourth part is a grip, which allows the practitioner to hold and maneuver the insert during use.
Each of these building blocks for the insert has an important role in the operation of the insert. The stack provides the necessary power to drive the tip. The acoustic transformer matches the impedance of the stack to the tip, provides amplification of the mechanical motion generated by the stack, and delivers coolant and lavage to the tip. The tip transfers the mechanical motion to the treatment area, which is often a tooth or root surface. It also directs the lavage or coolant provided by the connecting body. The grip not only allows the practitioner to hold the insert but is a key component since it attaches to the acoustic transformer. It is therefore critical that the grip connect at a nodal point for the motion along the length of the acoustic transformer. Attachment to a point not on the node will dampen the motion and reduce the available power at the tip of the insert.
Cavitron® Corporation introduced ultrasonic inserts to the dental market in the late 1950's. The first inserts were called “P” types to differentiate them from the cutting inserts first used for cavity preparation. Similarly as used today, the basic structure and design of these first inserts included four basic components: a laminated stack of a magnetostrictive material, a working tip, an acoustic transformer or connecting body that connects the tip to stack, and a grip allowing the practitioner to hold the insert during use.
These original Cavitron® designs adapted a surgical steel tube to deliver water to the treatment area. Straight Permanickel® laminations were used to generate the mechanical energy. An acoustic transformer amplified the mechanical motion generated by the stack, and a metal grip was attached to the connecting body using a compressed O-ring.
Over the years, improvements have been in the areas of water delivery to the treatment area. U.S. Pat. No. 3,930,173 to Banko and U.S. Pat. No. 5,567,153 to Foulkes et al. describe examples of the use of non-concentric holes in the tip of the insert. A swivel feature that allowed the practitioner to more easily rotate and adapt the tip along the line angles of teeth is described in U.S. Pat. No. 6,716,028 to Rahman et al. This design forced a compromise between the ease of rotation (generally rotational torque below 1 in-lb.) and leakage. At lower torque levels, the risk of coolant water leakage at the point of rotation was increased. Some designs utilize the slippage of the O-ring that seals the insert in the handpiece. These designs use a traditional toroid O-ring that seals both the handpiece-insert interface and provides a low torque rotation. They typically allow rotation within the gland area of the O-ring but are at the mercy of O-ring quality, dimensional tolerances of the O-ring, and handpiece dimensions to provide a good seal over all operating conditions.
Early inserts also had the disadvantage of loosening of the attachment of the grip to the insert after several sterilization cycles. Retightening was possible but alignment of the water tubing while retightening was problematic. In addition, the tightness of the capture mechanism often resulted in instability of the grip in both the lateral and longitudinal planes. Later designs used surface indentations to minimize loosening, but these designs did not address the problems of rotational and axial movement of the grip during use. U.S. Pat. No. 3,956,826 to Perdreaux describes a method of capturing the connecting body inside a cavity in a resin grip. While this eliminates rotational and axial instability, it results in a hard mount of the connecting body at the nodal point. The end result is an improvement in the stability of the grip but degradation in the tip motion caused by the hard mounting of the grip to the connecting body. A hard mount also increases the transfer of ultrasonic energy from the connecting body to the grip.
The original stack configuration for the Cavitron designs was a flat lamination without any added rigidity either along the length of the stack or in the cross-section. Some manufacturers added a “c” shape to the stack but this added little rigidity to the stack assembly, while other manufacturers added a “v” shape to the stack and glued the laminations together. While this approach added rigidity to the stack, bend angles less than 100 degrees introduced increased stress to the laminations and moved the center of gravity of the stack assembly off the concentric line of the stack-connecting body junction.
The stack assembly in typical ultrasonic dental inserts is brazed at both ends. The distal end has an end-ball attached to hold the ends of the laminations together and provide a rounded surface to minimize any damage to the handpiece during insertion and removal. U.S. Pat. No. 5,980,251 to Sullivan et al., for example, describes a method of creating a conductive connection of brazing material (silver solder) at both ends of the stack. First, this is counterproductive in that it increases the losses in the stack assembly during use. Second, it decreases the frequency of resonance of the stack assembly because of the lower sound velocity of the brazing material compared to the Permanickel® laminations. Third, the phase shift of the feedback signal in ultrasonic systems is adversely affected, especially with regard to those systems employing motional or velocity feedback.